The present invention relates to a process for producing an acoustic diaphragm made of carbonaceous material. More particularly, the invention relates to a process for producing an acoustic diaphragm of carbonaceous materials having light weight, high elasticity, fast sound transmission velocity and excellent rigidity as compared with a conventional diaphragm used as a speaker and a microphone. Diaphragms produced by the new process exhibit less deformation due to an external force, a small sound distortion, wide sound reproduction range, distinct sound quality, and are suitable for digital audio applications.
It is generally desired to satisfy the following conditions for a diaphragm intended for use as a speaker and as a voice coil bobbin:
(1) small density,
(2) large Young's modulus,
(3) large propagating velocity of longitudinal waves,
(4) adequately large internal loss of vibration, and
(5) stability against variation in the atmospheric conditions, and
(6) no deformation nor change of properties.
More specifically, a material intended for use as a diaphragm is required to reproduce sound in high fidelity over a broad frequency band. To efficiently and distinctly produce such sound quality, the material should have high rigidity, no distortion such as creep against external stress, and as a high sound propagating velocity. In order to further increase the sound velocity from the equation of EQU V=(E/p).sup.1/2
where V is sound velocity, E is Young's modulus and p is density, the material should have a small density and a high Young's modulus.
Conventional diaphragm materials include paper (pulp), plastic, and can further contain glass fiber and carbon fiber compositely mixed with the base materials. These materials can also be processed with metal, such as aluminum, titanium, magnesium, beryllium, or boron, metal alloy, metal nitride, metal carbide, or metal boride. However, paper, plastic and their composite materials have a small Young's modulus and small density. Thus, the sound velocities of these materials are low. Vibration division occurs in a specific mode and the frequency characteristics of these materials in the high frequency band are particularly low, resulting in difficulty in producing distinct sound quality. In addition, these materials can be affected by external environments, such as temperature and moisture, which cause deterioration in the quality and ageing fatigue. On the other hand, although metal plates of aluminum, magnesium, titanium, possess sound velocities which are faster than paper or plastic, they have small E/p values and small internal loss of vibration values. These materials exhibit either sharp resonance phenomenon in a high frequency band or ageing fatigue, such as creep, can occur.
Beryllium and boron provide excellent physical properties. Squawkers or tweeters which use beryllium or boron as diaphragms can extend the sound reproduction limit to audible frequency bands or higher, thereby correctly producing natural sound quality without transient phenomenon in the signals in the audible band. However, these materials are scarce natural resources and very expensive, and present difficulties in industrial machining. In particular, it is difficult to produce speakers of large size from boron and/or beryllium.
In addition to these materials, there have been experiments to obtain diaphragms made of carbonaceous material due to large E/p value of carbon materials:
(1) a method for carbonizing a resin sheet or film to graphite only;
(2) a method for shaping and carbonizing a composite material of resin and various carbonaceous powder to graphite, and
(3) a method for carbonizing carbon fiber-reinforced plastic to graphite.
Since method (1) has a small carbon yield due to the plastic material employed, a precise product is difficult to obtain and a product having high Young's modulus, like graphite or carbon fiber, cannot be obtained from carbon made of plastic.
Method (2) can produce a product having a high Young's modulus as compared with method (1) by using graphite or carbon fiber, but since it uses various resins to improve moldability, the ratio of the resin carbon to the calcined material is large, thereby causing the Young's modulus of the carbon fiber or graphite to decrease.
In method (3) only the plastic portion is baked and contracted. When the carbon fiber-reinforced plastic is calcined, numerous fine cracks occur among the carbon fibers so that a product in which the carbon fiber and the resin carbon are integrated without defects cannot be obtained. Therefore, this method has such a disadvantage that the function of the carbon fiber is lost.